/*
 * Background helper
 *
 * Examples:
 *   bg()
 *   bg(#fff)
 *   bg(#fff, 0.2)
 *   bg('image.png')
 *   bg('image.png', #fff no-repeat)
 */
bg() {
  $_done = false;
  $_length = length(arguments);

  // bg(<rgba>, <unit>)
  if ($_length == 2 && type(arguments[0]) == 'rgba' && type(arguments[1]) == 'unit' && arguments[1] <= 1) {
    background: rgba(arguments[0], arguments[1]);
    $_done = true;
  }
  return if $_done;

  // bg(<rgba>)
  if ($_length == 1 && type(arguments[0]) == 'rgba') {
    background: arguments[0];
    $_done = true;
  }
  return if $_done;

  // bg(<string>, ...)
  if ($_length > 0 && type(arguments[0]) == 'string') {

    $_other-values = arguments[1];
    background: url(arguments[0]) $_other-values;
    $_done = true;
  }
  return if $_done;

  background: none;
}